Update endpoints to support api.na.bambora.com platform

[k-nights]

Updated src/Beanstream/communications/Endpoints.php:

• Included support for na.bambora.com endpoints when $platform = 'api' (new default).
• Preserved backwards-compatibility with beanstream.com endpoints when $platform = 'www' (old default).

Updated src/Beanstream/Configuration.php:

• Changed default $_platform value to 'api' from 'www' for when the Gateway object is created without a $platform value.

Updated src/Beanstream/examples.php:

• Changed default $platform value to 'api' from 'www' which will send example requests to the api.na.bambora.com endpoints. Also added a comment to assist with TLS 1.2-Only testing.

Updated README.md:

• Added information related to backwards-compatibility with the beanstream.com endpoints.
• Added information related to TLS 1.2-Only integration tests.
• Added HTML formatting by wrapping result in <pre></pre>